Multiple signatures - why don't they work ?
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Multiple signatures - why don't they work ?
by ABU-7
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It's more than one week I'm thinking about multiple signatures.
Bruno Lowagie demonstrated in an ample tutorial [http://itext.ugent.be/articles/eid-pdf/index.php] how to add signatures : "approval" signatures and "certification" signatures. Unfortunately, that tutorial ended showing the inability to add an approval-signature to a certified document I think there's a bit of confusion about these two kinds of signatures, and moreover I believe there's an error in the current iText library (1.4.5) preventing to move forward. ---- concept section ---------- Just to provide a sound basis to the following discussion, here is a short recall to terms and concepts ... * [from "Digital Signatures in the PDF Language" - Adobe Dev.Tech. Note] Certification-signatures are almost identical to approval-signatures. The difference include: * a cert-signature is always the first signature applied to a doc. * an entry is made into a root dictionary that indicates that the signature is a cert-signature. When the originator (the author) makes a certification-signature, he also must specify what changes are allowed, choosing one of three levels of modification permitted: 1) no changes 2) form fill-in only 3) form fill-in and commenting --- end of concept section ---- Now, let's take back to the discussion ... As demonstrated by Bruno in its above cited tutorial, we know we can add many approval-signature to a document. These signatures are progressively appended within the PDF structure, and the resulting document is made of incremental revisions that can be examined separately. Since a new approval-signature is not "merged" with the originally signed-document but it is simply "appended", it means that a new approval-signature does not "invalidate" the previous ones ! A new signature "extends" the original document plus the previous signatures. So, at this time we can ask: "why the hell, in practice, a new signature invalidates a certification-signature" ??? Well, the answer is that a new signature does not invalidate a certification-signature, i.e. it does not "break" that signature but, it "breaks" the intentional limitations the author set for that document. (Here you can go back reading the "three levels of modification permitted"). I think we should pay attention (and respect) to the author's intentions: ... If you add a paragraph, some text or graphics over a "certified" document (..you know how to do it with iText ...), you could (even maliciously) deeply change the original 'meaning' of the document; the resulting doc could be very different from the author's intentions. In this case, should the author's certification (the cert-signature) be still valid ?? NO. This is not what the author provided. Therefore, when an author certifies a document, he/she also enforces some limitations to subsequent changes. Now we should restart from those "three level of modification permitted" if we want to add multiple signatures It is obvious that a certification-signature (level 1) does not allow ANY further signature; it prohibits ANY change! On the other hand, with a certification-signature (level 2/3) we "could" make next approval-signature but, be careful ! You are not allowed to ADD a signature, you are only allowed to FILL an existing signature field. To conclude, if the author provides a "certified" document with some blanc signature fields, then you are allowed to FILL-IN those sign- fields without invalidating the author's intentions. ----- Just to go back to the current state of the iText library (1.4.5), you can verify that there are two problems ... The first one is with the "addDocMDP" method (within the "PdfSignatureAppearance.java" file) "addDocMDP" allows certification-signatures with "level 1" permission only ! There should be a way to specify level 2 or 3, too. This is not difficult to fix, but there is the second problem ... The second (BIG) problem in "PdfSignatureAppearance.java" is that when you add an approval-signature over an existing blanc signature field, you *apparently* fill that field, but actually the following steps are performed: * create a duplicate of the signature field * fill it with the 'appearance' stuff * delete the original field * add the new field * .... * sign the document You can get a quick confirm of this behaviour by opening a signed document with AcrobatReader: [ http://itext.ugent.be/articles/eid-pdf/pdf/EIDFormInvalid.pdf ] Look at the "Signature Property" related to the first signature (it is a certification-signature), go to the "Document" tab and press the "Show changes" button. Now you can read details (this is a rough translation from Italian): * added "Signature2" field at page 1 * deleted "Signature2" field at page 1 * field "Signature2" at page 1 filled-in * 2 changes (????) ====================== Just to resume: **1** If we want to add multiple-signatures to Certificated-documents, we need the original certificated-doc were created with a "permission-level" equal to 2 or 3( level 1 means "NO change allowed" ). Note that the next signatures should FILL the originally provided (blanc) signature fields (i.e. No one is allowed to ADD a new (signature) field, you are only allowed to FILL those fields the author's provided ) This is easy to do with just a little internal patch in "PdfSignatureAppearance.java" **2** The actual implementation of "PdfSignatureAppearance", does not FILL an existing signature-field; it ADDS a clone (hiding the original). Although the "apparent" result is valid (i.e. ALL the signatures are valid), this operation (so implemented) breaks the special meaning of the certification-signature (remind the 3 "permission-levels" ) set by the author. ====================== I hope my arguments may help to find a solutions. Aldo Buratti Naviga e telefona senza limiti con Tiscali Scopri le promozioni Tiscali adsl: navighi e telefoni senza canone Telecom http://abbonati.tiscali.it/adsl/ ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642 _______________________________________________ iText-questions mailing list iText-questions@... https://lists.sourceforge.net/lists/listinfo/itext-questions |
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